Publish/subscribe (pub/sub) is an asynchronous messaging paradigm. In a typical pub/sub system, publishers post messages to a broker and subscribers register subscriptions with that broker. In a topic-based system, messages are published on particular “topics” which are used by the broker to identify matching subscribers. Subscribers in a topic-based system may receive all messages published on the topics to which they subscribe and all subscribers to a topic may receive the same messages (subject to filtering and assuming the subscriptions are active). Pub/sub is a very valuable paradigm, but not easily applied to streams of data, especially where the streams are already using an alternative predefined standard protocol.
Currently each pub/sub message is routed according to properties of the message itself (typically a topic name as mentioned above, but sometimes the content of the message). As each message arrives, the broker analyzes the message and determines the correct set of subscribers, and a second step distributes the message accordingly. This system is asynchronous, and does not easily support interfacing with other data communication protocols.